How to Calibrate Your Holley Carburetor for Mileage

So far I have discussed powerrelated calibrations of a Holley-style carb almost to the exclusion of all else. But in this day and age, I feel that we should only burn the minimum amount of fuel possible while achieving our goals in other quarters. Sure, I like a street performer that can make a 12-second pass, but I like it a lot better if it also does 25 mpg instead of the usual 15 to 18 mpg. Good, even very good, mileage figures are possible with a Holley carb. However, it’s not all about the carb’s basic design, but the selection of a suitable spec in the first place. It’s also about calibrations and how the carb is used. In this chapter I discuss what you need to know to get good mileage from a Holley-equipped engine.

SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://musclecardiy.com/performance/how-to-calibrate-your-holley-carburetor-for-mileage/

Every Holley has the potential to be a fuel miser, but achieving ultimate success means making a wise initial choice. A 600 vacuum secondary, such as this unit (PN 0-80457SA), was used on one of my Chevy 383 small-block budget builds with the intent of making torque and mileage the prime requirements. The result was 365 hp, 488 ft-lbs, and almost 22 mpg in a 1990 Sierra truck with a 5-speed transmission. All this came with a 13.9-second ET and 97-mph trap speed at the drag strip.

Mileage Makers and Breakers

At this point, I need to address a number of things pertaining to good mileage so you don’t end up going in the wrong direction.

Lean Mixture

The first thing on the list is to set up the engine so it burns as lean a mixture as possible. Follow this by calibrating the carb to deliver that “lean as possible” mixture. As counterintuitive as it may seem, the leanest mixture before misfire occurs is not the best for mileage. One that has just a trace of a miss every once in a while is better, but it is too lean from the emissions standpoint. There’s a fine dividing line between the two. During my in-depth efficiency tests for mixture versus fuel usage, I went to that minor miss point and came back to where a miss was extremely rare. This gave the cleanest exhaust while giving away only a fraction of a percent of fuel economy.

Venturi Size

Along with the leanest of mixture ratios, the mixture arriving at the cylinders, ideally, needs to be completely vaporized. If the rest of the engine is mileage oriented to at least a reasonable degree, a suitable mixture quality is not hard to achieve. The first step is to make sure that the fuel is well atomized as it leaves the booster. Achieving this entails having primary venturis that are definitely not oversized.

This Ford 302 with Scat stroker crank for 347 inches was a dyno mule for three months of testing carbs and manifolds. I ran about every brand of carb I could find, including Barry Grant (shown), Edelbrock, and Holley. It pumped out about 562 hp on a single-plane intake. However, the best drivability and fuel efficiency at part throttle were seen on a dual-plane intake and a vacuum secondary carb. With perfect street manners and a smooth 620-rpm idle, this 347 made 472 ft-lbs and 532 hp.

Booster Signal

The venturis need to be paired with a booster that has a high gain and good fuel-shearing properties. An example here is a main venturi in a 4150-style carb that is no bigger than, say, 1.4 inches paired with one of Holley’s high-gain annular discharge boosters. By creating a large signal, there is a need to increase the size of the air corrector in relation to the main jet size. When this is the case, the fuel emulsion in the emulsion well has a higher percentage of air in it by volume. This produces a more finely atomized fuel discharge from the booster. The smaller the mixture droplets, the greater the area they have in relation to their volume. This causes them to evaporate quicker.

However, there is a point at which the fuel is so finely atomized that it turns into a vapor and delivers no measurable advantage. This point is reached when the fuel droplets are 5 microns or smaller. Getting droplets down to this size is not easy so, in conjunction with efforts toward fine atomization, you need to employ other tactics as well.

Secondary

Under normal driving conditions, a typical V-8 has way more power than is ever needed with just the primary barrels in operation. This fact implies that a vacuum secondary is a far better choice for a street-driven machine than a mechanical secondary. Just in case you feel there may be a loss of performance from a vacuum secondary, let me assure you a correctly set up 650 vacuum secondary carb can make exactly the same top-end power as a 650 mechanical carb.

But that’s not the end of the story. At low RPM (from about 1,000 to about 2,500 to 3,000 rpm), the vacuum secondary more often than not shows better torque figures than a mechanical secondary carb, unless the mechanical throttle is driver modulated to limit the amount of venturi area presented to the engine. The lesson to learn here is that although racers don’t use vacuum secondaries they are often best when it comes to area under the power curve, drivability, and street fuel efficiency. (See Chapter 9 for more about the advantages of vacuum secondaries.)

Vaporizing Factors

After atomized fuel is discharged from the booster, it is subjected to manifold vacuum and heat. These play an important role in effective ignition and combustibility.

Intake Manifold Vacuum

You should have a decent amount of intake vacuum, which is at least 10 inches Hg under the cycle parameters expected during economy driving. This means that almost all, if not all, the fuel is vaporized by the time it gets to the intake valve. Effectively vaporizing the fuel also means that inter-cylinder mixture distribution tends to fall directly in line with the air distribution. This results in virtually the same mixture being delivered to each cylinder.

Intake Manifold Heat

Intake manifold heat also evens out mixture distribution and improves combustibility. By adding heat, you increase the volume of the intake charge and make a small reduction in the amount of residual exhaust that remains in the combustion chamber during the overlap period. This means slightly less mixture dilution and results in a charge that is marginally easier to ignite. Just how much heat is required can vary from engine to engine. Comprehensive tests on just one engine showed that some heat is good, but more can be too much. One scenario included ambient heat and whatever heat-soak was received from the rest of the engine. The next scenario utilized a coolant-heated intake that held the manifold temperature to between 180 and 190 degrees F. The last scenario was with an exhaust-heated intake.

The worst fuel efficiency came in the first scenario in which only ambient heat and heat-soak were involved. The middle efficiency was achieved with the exhaust-heated scenario. The best was from the scenario with a coolant-heated intake. For a performance-minded engine builder the middle scenario was a good compromise as it costs, on a typical 350-inch V-8, only about 5 hp over an unheated intake, as opposed to about 10 to 12 hp from an exhaustheated intake.

Enhancing Combustion Conditions

The next thing to consider is what the engine needs to turn your carburetion efforts into the best mileage results. In Chapter 1, I pointed out that most mileage gains come from the engine and not the carb itself, other than having it suitably calibrated. So let’s look at what the engine has to offer regarding potential mileage gains.

Compression Ratio

An engine’s compression ratio (CR) is the number-one mileage maker, or fuel-efficiency factor. To see how this is so we actually consider the expansion ratio (ER), which is the other side of the CR coin. When the CR is higher, the expansion ratio is higher. When the CR (and consequently the ER) increases, the amount of energy extracted from the cylinder pressure generated increases. In effect, fuel is utilized more effectively. (For a full explanation read my best-selling CarTech book David Vizard’s How to Build Horsepower.) Ultimately, fuel octane limits the amount of compression that can be used, but there are other things that can help enhance the CR situation a little beyond what the engine can stand at WOT.

Ignition Timing

You really need to understand the importance of having the optimum ignition timing for the mixture ratio and all its attributes, such as temperature, pressure, etc., that exist at the time combustion is initiated. If that timing is not set optimally your quest for mileage is compromised. Let’s consider the detonationlimited CR at WOT. If the engine took, say, 32 degrees of total timing advance to make the most horsepower, but the fuel put it into obvious detonation, you can save the day by backing out some timing.

Usually, in a situation where the fuel is a 3- or 4-octane number less than wanted, or the CR is 1 to 2 ratios more than the fuel can stand, retarding the WOT ignition advance by 2 to 4 degrees is the answer. However, the lower the cylinder pressure, the lower the octane value needed to stave off detonation. So, at part throttle, an engine with a little more CR than the fuel tolerates at WOT benefits from the extra CR at part throttle, where it is still a ways off from the WOT detonation limit. With some lean-burn tests, the timing for a 60-mph cruise is as far advanced as 55 degrees. Don’t be surprised by the amount of advance. The fastest burn is achieved when the mixture is a little on the rich side. Going richer or leaner slows the burn. The target is to have the cylinder develop peak pressure about 15 to 20 degrees after TDC on the power stroke. To make that happen, a very lean, slow-burning mixture needs a lot of advance.

Cylinder Fill

A certain amount of horsepower is needed to push a vehicle along at a given speed. Either a lesser amount of torque and a higher RPM or a greater amount of torque and a lower RPM can develop that power. For example, let’s say you want 60 hp from a typical small-block V-8 to push the vehicle along the freeway at, say, 70 mph. This could be done by making 60 hp at 2,000 rpm with the throttle open far enough to make 157 ft-lbs. Or it could be done by letting the engine turn 3,000 rpm at 60 mph, which at that RPM, means it develops 105 ft-lbs. The higher the RPM, the less efficient the development of that power becomes. This is because the cycle efficiency drops due to a less-efficient expansion cycle, and the fact that more of the power generated within the cylinder is being used to overcome internal engine friction.

A high-output ignition system is paramount for achieving good fuel mileage, as are the programmable advance curves of this Crane ignition system. The most important aspect, however, is the vacuum-advance system (arrow).

Let’s assume your vehicle can reach its true maximum speed for the power it has while in a 1:1 high gear. If you add an overdrive gear, which in effect over-gears the vehicle so it is actually faster in a lower gear, the mileage can improve dramatically. My tests in this area demonstrate the value of a high final drive. When the final-drive ratio of a mileage test vehicle was increased by 20 percent, the mileage at a 60-mph cruise improved by 19 percent. The idea here is to pick an overall cruise-gear ratio that is sufficiently high so that you have to open the throttle quite a ways to make sufficient engine torque to push the vehicle to the desired cruise speed.

Diesels are well known for their high fuel efficiency capabilities. This is partly due to the fact that they have very high compression ratios (20:1 is typical). They also do not have a throttling system on the intake. In effect, the throttle is wide open all the time. Power demand is controlled solely by the amount of fuel injected into the cylinder at the top of the compression stroke. What you are trying to do is emulate a Diesel’s power stroke. To do that, you add as much throttle (and consequently air) in the cylinder with the least amount of fuel. With a higher gearing, the amount of power developed within the cylinder suffers less frictional loss because the RPM is lower. The greatest frictional loss within an engine is due to the piston and ring friction on the cylinder bores. For this reason, if you are building an engine from scratch, it’s necessary to research rings and select a ring pack that is likely to have the minimum friction.

The narrower the rings, the less friction they have. The ring-to-bore preload is also a factor, and many specialty ring companies offer lowtension rings. The “low tension” part of the ring pack is usually the oilcontrol ring. Be aware that to make significantly less oil ring preloads work it is necessary to pull some vacuum on the crankcase. Aside from a correctly prepared bore finish, lower pre-load rings may require a vacuum of at least a couple inches of mercury to be pulled on the crankcase.

The Spark

Another critical factor for good fuel economy is the spark. The leaner the mixture becomes, the harder it is to effectively light it off. The difference between the spark required to fire a full-power rich mixture and a 20:1 economy mixture is like night and day.

Throttle Adjustment

On one occasion, my high-tech engineering friend David Ray and I managed to successfully light off a 22:1 mixture ratio and transition from that to a full-power mixture ratio with no misfire. I mention this because there is a transitional issue to address here. Under steady-state conditions on the dyno, it is possible to slowly lean out the mixture while equally slowly adjusting the throttle to maintain a given power output and achieve very lean burn without a misfire.

However, the moment the throttle is opened at any sort of typical rate the engine simply signs off. The reduction in manifold vacuum due to an increased throttle opening causes some of the fuel to drop out of its vaporized state and form as a liquid on the manifold walls. This leans out the air/fuel ratio of the mixture flowing in the port itself and results in an unburnable charge. In effect, the leaner the mixture you can run at steady state, the more critical the accelerator pump function becomes.

Spark Energy

The spark must be big, intense, and hot to successfully fire a superlean mixture. With an optimally prepared charge in the combustion chamber, near fully effective ignition can be achieved with as little as 0.2 millijoules of energy at stoichiometric mixture ratios. When a richer fullpower mixture is used, this energy requirement can increase by a factor of 10. The same goes for moderately lean mixtures. If you are dealing with mixture ratios leaner than about 16:1, the spark energy required can go even higher.

When it comes to ignition systems, there is almost no such thing as “overkill.” I use MSD systems on most of my race engines and my mileageoriented street engines.

There is a lot more science in coils than you might think. Though the design of the coil is important, one of the most influential factors for generating an effective spark is how fast the current to the primary winding is cut. That is a function of the ignition box

This MSD distributor is a great unit to use on a race engine, but not on a street engine where mileage is required. Why? Because it does not have a vacuum advance. Don’t listen to anyone who tells you vacuum advance is not really necessary, even for a street machine.

What we are considering here is spark energy values for a well-prepared mixture in an effective combustion chamber. The question to ask is, How closely does a particular application emulate a laboratory test engine? In reality you can only guess. What this means is that your ignition system needs to have plenty of overkill capability. This is certainly one of the few areas where the Stroker McGurk Syndrome actually pays off. According to Stroker, “If some is good, more must be better, and too much must be just right.” When it comes to effectively igniting lean mixtures, the amount of energy required can escalate to typically unprecedented values. So, when selecting an ignition system, overkill is more than just okay.

Duration

If everything is good in terms of mixture quality, mixture motion, etc., what happens spark-wise in the first few hundred nanoseconds to a micro second pretty much dictates what happens from there on out. (There are 1,000 nanoseconds in a micro second, 1,000 micro seconds in a millisecond, and 1,000 milliseconds in one second.) A nanosecond is a very short time. Because of the energy distribution pattern during the entire spark, it is only the first part of the spark that does any real mixture ignition. You need the energy in the initial part of the spark to be as high as possible. Also, because you do not know exactly how ignitable the charge is in your hot rod engine, you often find that firing the spark multiple times or having a long-duration spark is also a benefit, especially at low speed.

Spark Plug Gap

The energy in the initial spark phase is more or less directly related to the spark plug gap. The bigger the gap, the more energy the first and most critical few nanoseconds of the spark has. Countering the ability to fire the gap in the first place, the bigger the gap, the higher the ionization voltage. That’s the voltage required from the coil to break down the resistance of the air/fuel mixture between the plug electrodes.

If a plug cable shows a resistance in the thousands of Ohms range, it is for one of two reasons. It could be because of a broken down core, or it could be a carbon string cable. Although carbon string is okay when new, it does not last. It’s best to use spiral-wound core cables.

Multi-electrode plugs (such as the one seen here or similar to the Split-Fire plug) are mostly a sales gimmick. The only advantage of multiple-side electrodes is extended life, as spark erosion is shared among four electrodes instead of one.

The electrode on this plug has been converted to the lower voltage/ higher energy design in Figure 5.11.

Plugs with the pin-style platinum center electrode can last up to 60,000 miles and work well with a high-output ignition system.

Here is a plug electrode design I developed in the early 1990s. Its main asset was that it produced a very high energy spark while requiring a considerably lower ionization voltage.

If you are going to get the timing right, you need to find TDC and make sure it is established accurately.

All other things being equal, a prepped wider-gap plug (right) gives a spark of as much as 20 percent more energy in the initial light-off phase than a conventional plug (left).

To run the leanest air/fuel ratio possible, use an ignition system capable of the greatest voltage possible without outpacing the insulation of the plug insulator, the plug cables, and other ignition components. Remember, you are looking for a spark at the plug gap, not between the exterior of a plug cable and some random location on the block.

Temperature

Another factor of importance is the temperature of the spark. A typical spark from an inductive ignition system is about 3,300 to 3,800 degrees F. My own dyno tests have strongly indicated that even a fullpower rich mixture benefits from a higher spark temperature than that. A capacitive system typically develops a spark that is hotter but also shorter in duration than an inductive system. However, there are systems (other than the extremely high dollar laser ignition systems) that can bring about much higher plug energy levels and do so with significantly higher temperature.

System Sources

To get the job done for a typical cruise RPM (about 3,000), a good place to start is by looking through MSD’s catalog. The basic MSD 6 gives a strong spark and multi fires up to about 3,000 rpm on a V-8 and to higher RPM on engines with fewer cylinders. Do the multiple sparks pay off? For a typical, hopped-up production V-8 the answer is yes in nine out of ten cases.

No matter how high-tech you get with the ignition system and any data acquisition system, remember that one of the besttune-up devices is a simple vacuum gauge such as this Autometer unit.

Here is the difference between asingle GM HEI spark (left) and a Plasmaboosted one (right).

My Canadian friend Bill Ball ison the crew of this Merkur XR4. On twoseparate occasions Bill tested the Okadaplasma system. The first time, it produced9 additional horsepower on the Merkur’sFord 302 small-block. The second time, itproduced 5 hp more than a conventionalignition system.

If the ignition system delivers only one spark per firing cycle, spark duration can also play a part in dictating the lean-limit misfire point. In many ways, a longer-burning spark can replicate what a multi-spark system does in the way of lighting off a very lean mixture. Crane’s system (directly competing with the MSD range of ignitions) does a very good job. I used one of them for a research project; it was a multi-spark system in which each spark had a longer than average burn time. The results were top line, but whether that was due to spark duration or spark intensity is hard to say.

Some Unconventional Systems

Still on the subject of spark, and justifiably so because of its strong influence on the leanest burn practical, let’s consider some unconventional systems for super-spark generation.

David Ray Experiment

My first involvement with a “way out in left field” ignition system was in the mid 1970s. I received a call from my friend David Ray, with whom I had been working and dyno testing on my Round America Economy Drive project. He had heard, through some mutual Cosworth engineer friends, of a system that had been presented to Cosworth for evaluation. It was apparently the brain child of a guy who would fit a Disney movie representation of a totally brilliant scientist, but one who was not quite in the same world as the rest of us. This guy had come up with a system that initially fired the plug with a super-high voltage and immediately the gap ionized it; it put a very highamp DC voltage across the gap. This turned the spark plug into something akin to an arc welder. Needless to say the plugs did not last long. But for an F1 race, if they were good for 300 miles, it was fine.

These are the results of just one chassis dyno test on a Ford Mustang. Thistest was on a 5.0 pushrod engine. Tests on the 4.6 Modular engines consistently deliveredgreater increases.

When David and I tested this, we rigged it up to see what the spark looked like in open air. We were amazed because the system was able to put a small bolt of lightning across a gap about a foot or so in size. When activated, the noise it made sounded like a pistol shot. On the Cosworth DFV F1 engine it was reputed to add 11 hp over the ignition of the day. The big problem was not with the system’s capability in terms of delivering performance, but in terms of safety. One shock from it, and you really were 100-percent dead! But the lessons learned were not lost on me. Testing this system demonstrated that sustained high energy and temperature can, in a less-than-perfect combustion situation, be key factors in firing mixtures as lean as 22:1; and that was in 1975 before the term “lean burn” was in vogue.

You’re probably asking yourself, Are there any systems that go at least part way to emulating this deadly system? The answer is yes. I am sure there are more than the ones I discuss next, but to save you the effort of hunting down such systems, let me run through the ones I have had experience testing.

Nology Engineering, Inc.

Other than the potentially deadly system that I tested with David Ray my first test of a “different” ignition was that of the Nology plug cables. They not only have a connection directly to the plug but an outer stainless-steel sheath around the plug cable connects to the block at some convenient point. Nology cables function like this: When the coil voltage to the plug builds up instead of firing the plug it charges the plug cable, which is acting as a capacitor. When the voltage reaches a very high value it fires across the plug in an extremely short time. Because the available energy is dumped into the plug gap in such a short time, the wattage increases dramatically by a factor of about one million. This extremely powerful spark is also at a very high temperature that is typically close to 50,000 degrees F.

When I first tested these plug leads, I felt it was a question of determining if the increase in spark size, intensity, and temperature offset the dramatically reduced duration of the spark. After much testing, the results showed that the vastly more aggressive spark, even though it was of an extremely short duration, paid off in increased power and lean-burn capability.

Okada Projects

The next system on my short list is the Ignition Solutions Plasma multi-fire system from Okada Projects. This system piggybacks onto whatever ignition system is already in use. Its capacitive/inductive mode of operation means it is extremely reliable; in the unlikely event of failure, it automatically reverts to the stock ignition. This unit not only produces a very hot, intense spark but multi fires the spark or sparks of the original system. So if you have, say, an MSD 6 system delivering four sparks, the Plasma Booster multi sparks each of the MSD’s four sparks.

I tested this system on two-, three- and four-valve engines and got positive power results every time. All the vehicles were Ford Mustangs. I saw the least gain on a 1990 Mustang 5.0. The 1990 Mustang 5.0’s baseline ignition system was already mildly reworked with a higher output coil and top-notch plug cables and reworked electrode plugs. There were only minimal gains up to the midrange RPM and an increase in output topped out at 5 hp on peak. At the other end of the scale I saw the most gains on the four-valve-percylinder Mustangs.

A Blue Phoenix system is shown in a high-speed video sequence of the developing plasma. It is far more energetic than thespark from even GM’s High Energy Ignition (HEI) system.

Because the system only took a few minutes to install it was good in terms of cost versus install time. As far as typical results from an ignition hop-up are concerned, it also scored well. Gains of this magnitude are usually only seen when the original system is mediocre. The results of my first tests on the 5.0 Mustang prompted me to test on some of Ford’s newer pony cars. The idea was to see if the Plasma Booster was capable of increasing output for an engine with an ignition designed to be a high-performance system. Ford’s Modular engines (4.6- and 5.4- liter V-8s) have a coil-over plug system, which was used with the intent of maximizing ignition capability for the best power and economy possible. After three tests on different Modular designs (two-, three-, and four-valve variants), I found the Plasma Booster to be an effective hop-up tool.

Blue Phoenix Ignition Systems

In many respects, the Blue Phoenix system is similar to the superhigh-output system that David Ray and I tested in the mid 1970s. Remember, the goal is to ionize the plug gap with a conventional highenergy spark and then, when the resistance drops due to the plug gap ionization, a DC voltage is superimposed on the gap. This produces a long-burn plasma arc that persists for 20 degrees or more of crank rotation. Sure this sounds exotic, but does it work? From the 1975 tests with the “deadly” system I knew that, at least in principle, the system works. Though hardly “tame,” this system was in effect a tamer version of the one from which Cosworth reputedly saw 11 hp more during F1 engine tests. I tested the Blue Phoenix system on one of my big-block Chevy builds. The test engine was a really well developed street engine of about 525 ci. The wet flow, swirl, and port velocity of the heads was ideally suited to the MSD multi-fire ignition system and its plugs and cables.

The combustion properties of this project engine were such that I could run 87-octane fuel (R+M over 2) with mean best torque timing without any sign of detonation. This indicated an efficient combustion process was being achieved. The Blue Phoenix system would have to prove the value of its long-burn plasma arc so it needed to show well. Rather than show you yet one more dyno curve I thought it better that you see the actual tests I ran with the guys at Terry Walters Precision Engines. As of the writing of this book, a video of the dyno test was posted on Youtube. When you watch this video, note the very slow idle speed on the test engine. This is a true street engine in all respects, including the fact that it was being run on 87-octane fuel. The final output was just shy of 800 hp.

Economy Tuning

The Round America Economy Drive project had some heavyweight backers and was highly promoted by Popular Mechanics in the UK. This project was centered around the original A-series British Leyland (now defunct) Mini. To measure fuel consumption as accurately as possible, we made up a system that employed solenoids, timers, and a very tall, skinny burette. Here is how it worked. Typically, the engine ran on fuel directly from the dyno cell’s fuel tank. After the engine had been stabilized at a particular road load and engine speed, a start button actuated the solenoid that swapped the fuel supply from the cell tank to the burette. At the same time, this started an electronic timer. The engine continued to draw fuel from the burette until it had run long enough to accurately establish how much fuel it had drawn out of the burette in, say, 60 seconds. At that point, I hit the stop button.

I was left with a very accurate measurement of the amount of fuel used in that amount of time. With diligence we were able to measure BSFC to a significant third decimal place. To test and make fine-tuning adjustments to this degree takes a really good dyno, a lot of time, and some very accurate fuel-flow measuring gear. As you can imagine that test procedure is not exactly a user-friendly way to go about tuning a Holleyequipped engine for economy. Fortunately there is an easier way. First, find a long stretch of flat highway; it needs to be truly flat for at least 2 miles and preferably 4 to 5 miles.

Next, get a Mity Vac vacuum pump/tester as shown in Figure 5.19. Install a vacuum gauge to read intake manifold vacuum. You can use a typical Autometer (or similar) vacuum gauge, but a commercial/industrial big-dial gauge is better because it can be read more accurately. Economy tuning can be done without a wide-band, oxygen-mixture measuring system, but I highly recommend using one. A good cruise control unit is also an asset. You also need an assistant with a pen and notebook to record the data you are about to measure.

To get the best results, it is important to understand what you want in the way of vacuum gauge readings. As far as spark timing goes, the higher the vacuum, the more efficiently your engine is burning fuel at the test speed. As far as mixture is concerned the opposite applies. As the mixture is leaned out, it becomes necessary to open the throttle more. This reduces the vacuum in the manifold. This may not be intuitive, but keep in mind that you want to get as close to a Diesel cycle as possible with your spark-ignition engine. Before getting into tuning details, let me remind you that getting any real economy figures means having vacuum advance on the engine. Without that, you can be sure that at least 30 percent of your engine’s cruise speed economy potential has been sacrificed.

Also, before starting the actual tuning, be sure to record the distributor or ignition system’s RPM-related (usually referred to as mechanical) advance. Plot this in increments of about 100 rpm from just over idle to about 3,000 rpm (or whatever the maximum cruise RPM is likely to be). This needs to be plotted out in large scale on a sheet of graph paper. Your tune-up assistant needs this during the tune-up procedure, so make it easy to read while riding in the vehicle.

Tuning Steps

After you have the required gear, follow this tuning procedure: First, install the vacuum gauge and the oxygen system. Next, disconnect the vacuum advance from the intake manifold, plug the port, and then connect the manifold end of the distributor vacuum advance tube to the Mity Vac vacuum pump. You can now advance the ignition, at will, from within the driver’s compartment. On the test stretch of highway, bring the vehicle up to the test speed. For a truck, such as our race car tow vehicle, I typically choose test speeds of 35, 50, and 65 mph. For a car, I use 35, 55, and 70 or 75 mph. Once you’re consistently traveling at the test speed, slowly actuate the Mity Vac pump to bring in vacuum advance. As the advance nears optimal, the vehicle tries to speed up, but the cruise control reacts by slowly closing the throttle to maintain the preset speed. (This needs to be done by manual throttle modulatiosn if your vehicle is not equipped with cruise control.) This causes the vacuum, at that preset speed, to increase. The ignition timing for the current speed and jetting is optimal when the manifold vacuum is highest.

Record the oxygen sensoroutput, manifold vacuum, RPM, and Mity Vac vacuum at the current test speed. Repeat the procedure for each of the test speeds.

Ignition Advance

You are developing a part-throttle ignition curve for the engine. So, back at the shop, open the hood and use the data you have just recorded to determine the advance at each RPM/Mity Vac vacuum reading. To do this, rev the engine to the test RPM, apply the recorded Mity Vac vacuum setting, and check the ignition timing.

The number of degrees you see is the timing required for the manifold vacuum. The Mity Vac does not provide the vacuum, rather it is just a means of adjusting the timing. Subtract the mechanical advance from the total advance. The result is the vacuum advance required at that engine speed and load/manifold vacuum.

At this stage, let us assume that the WOT mechanical/RPM advance curve is correct for the engine spec. You have just determined how much vacuum advance is needed on top of the WOT RPM advance. If the mixture was optimum, this is the advance characteristic you seek to get from either programming a computercontrolled ignition system or a regular distributor’s vacuum-advance canister.

Cruise Calibration

It is very unlikely that you hit the lean burn limit on your fi rst test but you should have recorded the air/fuel ratio at each of your cruise RPM speeds. With most out-of-thebox Holley carbs, the oxygen reading indicates a mixture around 14:1 to 15:1. Your next job is to lean out the carb’s calibrations, and that is done mostly on the idle fuel and air jets, with the main jet and air corrector coming in only at the higher speeds. After your initial test, lean out the circuits that control the mixture. You should be able to approach 17:1 relatively easily, but be aware that at these leaner ratios, the action of the accelerator pump and its tuning to meet the transitional needs becomes a lot more fussy.

Retest at the leaner carb settings according to the initial test. Again, you are using optimal timing to maximize engine vacuum. If the trend is toward the successful use of a leaner mixture, after optimal timing has been achieved with the leaner mixture, the amount of vacuum is a little less than with the previous test.

Basically it is a process of progressively leaning out the mixture and reestablishing the optimum vacuum advance curve at each step. As soon as the engine runs into lean misfi re, take a small step back and go richer with the mixture. When the engine is running into a lean misfi re, it starts to show readings leaner than expected. Remember, the oxygen sensor is looking at the oxygen in the exhaust. A misfi re puts a lot of unused oxygen into the exhaust. A tune up like this means your engine is delivering close to the best economy it is likely to get with its current spec. Now you must tailor the ignition to deliver the vacuum advance curve required to make the most of the lean mixture being delivered.

Written by David Vizard and Posted with Permission of CarTechBooks

GET A DEAL ON THIS BOOK!

If you liked this article you will LOVE the full book. Click the button below and we will send you an exclusive deal on this book.